very low frequency reproduction does benefit from high-sound-quality amplifiers, but nowhere near as much (or as noticeable) as the full-range of the mains. For those, I'd opt for an amp that was more sound quality oriented for the non-subwoofers in my system. Xtant, Zapco, US Amps, Tru, etc. if I had the inclination to drop the bux.
Honestly, I haven't heard a system running the new and much improved XX line from HiFonics. It's probably better than most anything in its price bracket. So, even for mains, it's not absolutely out of the question. For subs, though, it's really hard to beat the value of the HiFonics XX series, without a doubt.